This invention relates to a method and system for the near simultaneous analysis of multiple gases by means of Raman scattering wherein the Raman scattering sample is placed within the laser resonator and a single detector is utilized for quantitating the Raman signals of each gas being analyzed. The invention also relates to a system which does not utilize a spectrometer or spectrograph but rather employes a series of filters which reject the elastically scattered laser line while passing the particular Raman lines of interest. More specifically, this invention relates to a method and system for the detection of multiple respiratory and anesthesia gases by Raman scattering wherein the incident laser beam passes through the gas sample placed in the intracavity of a laser and a rotating filter wheel, containing filters specific for each Raman scatter line of interest, is used to transmit light onto a single appropriate detector for quantitating each specific Raman signal, and thus each gas.
The monitoring of respiratory and anesthetic gases as well as specific cardiac and pulmonary functions which in turn are based upon the uptake and production of specific gases has reached a high standard of technological advancement with the development of sophisticated sensors, transducers, and computers. These monitoring techniques enable quick diagnosis and treatment of unfavorable trends in the condition of a patient and lead to an improved survival rate, early extubation following surgery and a shorter time in the intensive care unit. Applications of respiratory and anesthesia gas monitoring include the measurement of anesthetic uptake, oxygen consumption, and carbon dioxide production These measurements lead to a more scientific basis for the administration of anesthesia. A breath-by-breath analysis of multiple respiratory and anesthesia gases of patients in the operating room, and of respiratory gases in intensive care and other critical situations can often facilitate diagnosis and treatment, anticipate and prevent the development of oncoming problems and otherwise provide instant data for physicians and other health care personnel to use in therapeutic situations. The same may be said of the breath-by-breath analysis of gas mixtures used for noninvasive determination of cardiac output and lung function.
Respiration monitoring of the critically ill patient is now available in intensive care units. Multiple bed sampling techniques make feasible the use of an expensive, multiplexed mass spectrometer because it can be shared among a number of patients. Since the unit is large and not easily moved from room to room, it is generally placed in a remote location and lengthy capillary tubes are used to connect the patients. This tube transport system increases the possibility of gas sample mixing, time delay, and disconnections and poses inherent limitations for use in anesthesia, critical care and medical research Mass spectrometry also has only limited flexibility in the study of gas mixtures. Alternatively, there are a variety of gas detectors based upon several different physical principles which, taken together, can measure anesthesia and respiratory gases. Their problems are: high aggregate cost, bulkiness and poor data integration into one comprehensive display of patient parameters.
An alternative proposed for use in monitoring several gases in critical care situations is based on Raman light scattering. The Raman light scattering effect relies on the interaction of monochromatic light with the vibrational/rotational modes of molecules to produce scattered light which is frequency shifted from that of the incident radiation by an amount corresponding to the vibrational/rotational energies of the scattering molecules. Since these energies are species-specific, an analysis of the various frequency components present in the Raman scattering spectrum provides chemical identification of the gases present in the scattering volume. The intensity of the various frequency components or Raman lines provides quantitation of the gases present providing suitable calibrations have been made. The relative sensitivity to the different gases remains absolutely fixed, eliminating frequent calibration requirements.
Raman techniques have been widely used for atmospheric monitoring and for combustion applications. Sensitivities better than 1 ppm have been demonstrated. Typical application of Raman scattering analysis coupled with computer assisted signal processing techniques is reported in Lapp et al., "Laser Raman Gas Diagnostics", Plenum Press, New York/London, 1974.
Raman scattering analytical techniques are also described in the patent literature. Chupp, U.S. Pat. No. 3,704,951 teaches laser Raman spectroscopy utilizing a sampling cell with a multi-pass configuration. A laser beam enters into the cell configuration of concave mirrors facing each other such that there is a multiple reflection of the laser beam between the mirrors to accomplish the required optical power density enhancement in the sampling area and subsequent signal enhancement. This device and accompanying technique is limited in that it provides for analysis through only a single detector. Hence, simultaneous monitoring of multiple gases is not possible. Moreover, this device is intended for use primarily with liquids and has only limited application for gases. Also, the alignment of the mirrors for optimal signal is exceeding delicate. Finally, the beam size in the sampling region must be quite small to maintain low sample volume and subsequently high signal response time. A multimirror approach makes this difficult, if not impossible, given the optics of such a system.
Hatzenbuhler, U.S. Pat. No. 3,807,862 also teaches a specific application of Raman spectroscopy in which a fluid sample is subjected to a laser beam and only a single Raman line is evaluated. In other words, there is no teaching of a technique for the determination of multiple gases.
Leonard, U.S Pat. No. 3,723,007 is drawn to a method for the remote sensing of gas concentrations through use of a high-energy pulsed laser and a mirror telescope, using a grid polychromator. This system requires a laser output in the 10 kW range and is unsuitable for general application. Moreover, the use of an expensive spectrometer presents an obstacle in the way of cost-beneficial production of the device.
A more recent and effective system for the simultaneous detection of multiple gases is taught in Albrecht, et al., German Pat. No. DE 27 23 939 C2. This patent also utilizes a multi-pass cell to constrain the laser radiation in a region between two concave mirrors for signal enchancement but utilizes an unpolarized laser beam to provide a 360.degree. monitoring geometry for the Raman scattered light. A series of six detectors, each accompanied by an interference filter comprised of one broad-band and one gas-specific filter, are provided to collect six separate Raman lines for the simultaneous monitoring of six different gas components. This method, while monitoring multiple gases simultaneously, requires six separate detectors including separate photomultiplier tubes and recording instruments. Such a complex system is bulky and expensive. Moreover, since the orientation of the six detectors described in the German patent could not be expected to exactly image in the same area, the acquisition of all gas concentrations could not be from exactly the same point in the gas flow stream.